It is well known to support a pair of car wash brushes having substantially vertical axes of rotation on booms or “knuckles” which can pivot about vertical axes of rotation adjacent to a car wash lane to engage and scrub the front sides and rear surfaces of a vehicle. It is typical to use a power cylinder which urges the brushes to the outermost or extended positions wherein they are closer to the center line of the wash lane and on appropriate command deactivate the cylinders and allow gravity to move the brushes back toward a retracted position on the boom or knuckle while the vehicle is passing between them. If the brushes are to engage and follow the rear surface of the vehicle it is necessary to reactivate the cylinder to urge the brushes toward the extended or outermost positions as soon as the vehicle is far enough forward to clear the brushes and allow them to move back out toward the center of the wash lane. It is particularly problematical to program the initiation of the outward re-extension of the brushes to boom or knuckle angle since vehicles of different width displace at least one of the booms or knuckles to a different extent as they pass between the wrap around brushes.